Monoclonal antibodies against CD20 are a highly effective therapy for multiple sclerosis (MS) currently in phase III clinical development. CD20 is commonly viewed as an archetypical B cell marker. However, a subset of human T lymphocytes (T cells) also expresses CD20. Presently, not much is known about the functional or pathological relevance of CD20-expressing T cells, but a possible involvement of this T cell subpopulation in autoimmune disorders has been suggested. CD20+ T cells can assume a pro-inflammatory Th17 phenotype in rheumatoid arthritis (RA) and MS, and increased numbers of CD3+CD20dim T cells can be found in peripheral blood (PB) of MS patients. Like CD20+ B cells, CD3+CD20dim T cells are effectively depleted from PB of MS and RA patients by the anti-CD20 antibody rituximab, which may, in part, be responsible for the effectiveness of anti-CD20 therapeutic strategies. Unpublished preliminary experiments suggest that CD3+CD20dim T cells in PB may be increased during MS relapses; CD20-expressing T cells are also present in CSF but an association with disease-activity has yet to be studied. Furthermore, next-generation deep T cell receptor -chain variable region (TCR-V) immune-repertoire sequencing suggests that identical CD20dim T cell clonotypes in peripheral blood and CSF may be involved in MS disease-activity. To our knowledge, no murine equivalent to human CD3+CD20dim T cells has been identified. However, treatment of mice with an antibody specific for MS4aB1, a murine CD20 homolog expressed on T cells, was found to ameliorate disease severity of experimental autoimmune encephalomyelitis (EAE), theoretically mimicking the therapeutic effect of rituximab-mediated CD3+CD20dim T cell depletion, in the absence of B cell depletion, in humans. The objective of this research program is to delineate the potential pathological involvement of CD3+CD20dim T cells in the immune pathology of MS. Methods: We will perform extensive phenotypic, transcriptional, and functional characterizations of CD20+ T cells in PB (Aim 1), to examine whether CD20+ T cells and/or other T cell subsets can provide an antigen-specific, immunologically active, and sustained connection between the periphery and CNS compartments (Aim 2), and to determine their prevalence in MS CSF during different stages of the disease (Aim 2) and compared to other neurological diseases (Aim 3).